Real-estate values and construction have peaked one to two years before a depression, and have stayed at peak levels until the onset of the downturn.
The historical evidence is consistent with the theory that speculative booms in real-estate prices and construction act as an impetus for the downturn itself.

For an explanation, see my article:
"The Business Cycle: A Georgist-Austrian Synthesis." American Journal of Economics and Sociology 56 (4) (October 1997): 521-41.

An updated explanation (2007) is in my booklet,
The Depression of 2008, by the Gutenberg Press.